In another thread, I've talked about my miserable failure cooking a pizza in a grill over a campfire. One suggestion was that perhaps I need to erect a "roof" of quarry tiles above the pizza to apply heat from above. I'm certainly not doubting that. I just don't understand it.

In my oven at home, I cook the pizza on the bottom rack and there is a lot of open space (14") above it, but it cooks just fine. Why would having stone a few inches over it help?

I did not read the other thread you mention, but I would guess there may be a problem in retaining heat in the grill when you open up the top. Your oven at home likely recovers from door opening more quickly than a grill. Grills also tend to have many gaps that heat can escape from as well. The tile above the pizza will serve as a heat source that will stay hot even if the top is opened for a while. The closer to the tile your pizza is the faster it will cook.